During electrolysis, for example, electric energy is converted into chemical energy. This is achieved through the decomposition of a chemical compound by means of an electric current. The solution used as electrolyte contains positively and negatively charged ions. Therefore, mainly acids, bases or salt solutions are used as electrolyte.
In the case of the electrolytic production of halogen gases from aqueous alkali halide solution—here represented by sodium chloride—the following reaction takes place on the anode side:4NaCl→2Cl2+4Na++4e−  (1)
The liberated alkali ions move to the cathode where they form caustic with the hydroxide ions produced there. In addition, hydrogen is formed:4H2O+4e−→2H2+4OH−  (2)The caustic produced is separated from the alkali halide, which is fed to the anode side, by means of a cation exchange membrane, and in this way separated from each other. Membranes of such kind are state-of-the-art and commercially available from various suppliers.
The standard potential at the anode which is generated by formation of chlorine when the above reaction takes place is +1.36 V, with the standard potential at the cathode being −0.86 V when the above reaction takes place. A cell design of such kind is known from WO98/55670, for example. The difference of these two standard potentials yields an enormous energy input which is required to perform these reactions. In order to minimise this differential amount, gas-diffusion electrodes (hereinafter referred to as GDE) are used on the cathode side no that oxygen is supplied by the system resulting in the following reaction at the cathode instead of reaction (2):O2+2H2O+4e−→4OH−  (3)The oxygen can be supplied as pure gas or by means of air. The resulting basic overall reaction of the chlor-alkali electrolysis using gas-diffusion electrodes is the following:4NaCl+O2+2H2O→4NaOH+2Cl2  (4)As the standard potential of reaction (3) is +0.4 V, significant energy savings are achieved by the use of the GDE technology as compared to the conventional electrolysis with the formation of hydrogen.
Gas-diffusion electrodes have been used for many years in batteries, electrolysers and fuel cells. The electrochemical conversion in these electrodes takes place exclusively at the so-called three-phase boundary. Referred to as three-phase is boundary is the zone where gas, electrolyte and metallic conductor coexist. To ensure that the GDE works effectively, the metallic conductor should also figure as a catalyst for the desired reaction. Typical catalysts in alkaline systems are silver, nickel, manganese dioxide, carbon and platinum. Great efficiency of the catalysts is ensured if their surface is large, which is achieved by fine or porous powders with specific surface area.
Problems in the use of such gas-diffusion electrodes as disclosed in U.S. Pat. No. 4,614,575, for example, are due to the fact that the electrolyte would penetrate into these fine-pored structures by capillary effect and fill them up. This effect would make the oxygen stop diffusing through the pores and would thus stop the intended reaction.
To ensure that the reaction takes place effectively at the three-phase boundary, it is required to avoid the before-mentioned problem by selecting the pressure conditions accordingly. The formation of a liquid column in a static liquid as in the case of the electrolyte solution causes, for example, the hydrostatic pressure to reach its highest value at the lower end of the column, which would intensify the above-described phenomenon.
As known from the relevant literature, this problem is solved by using falling-film evaporators. The electrolyte, such as caustic soda solution NaOH or caustic potash solution KOH, for example, is caused to pass through a porous material between the membrane and the GDE, thus preventing the formation of a hydrostatic column. This is also referred to as percolation technology.
WO 03/42430 describes such an electrolysis cell, which uses this principle for the chlor-alkali electrolysis reaction with an oxygen consumption reaction. In this, the oxygen is separated from the porous material by the gas-diffusion electrode and the oxygen and the porous material—the percolating agent—are pressed together by means of a conductive supporting structure and a conductive flexible spring element.
Such a principle is also known from DE 102004018748, for example. Here, an electrochemical cell is described which consists of at least one anode compartment with an anode, a cathode compartment with a cathode and an ion exchange membrane arranged between the two compartments, with the anode and/or cathode being a gas-diffusion electrode, a gap being arranged between the gas-diffusion electrode and the ion exchange membrane, an electrolyte inlet above the gap and an electrolyte outlet below the gap as well as a gas inlet and a gas outlet, the electrolyte inlet being connected to the electrolyte receiver and consisting of an overflow.
However, the objective of the use of the gas-diffusion electrode in the electrolysis apparatus described is not only to allow the catalytic oxygen consumption reaction. The electrode is also expected to ensure the separation of electrolytes and gas on both sides of the GDE. For this purpose, it is absolutely essential to provide the gas-diffusion electrode with gas-tight and/or liquid-tight sealing by the fixing method selected in order to ensure—especially after entry of the electrolyte into the cell—that the electrolyte is routed along the gas-diffusion electrode as specified and does not reach the electrolyte outlet of the electrochemical cell via areas not adequately sealed and thus constituting alternative routes, consequently not being available for the reaction.
As gas-diffusion electrodes are subject to ageing and thus to wear, they must be replaced after a certain operating period. Prior art provides for the welding of the gas-diffusion electrodes to the cathode compartments, which makes replacement very laborious.
This is described, for example, in DE 103 30 232 A1. Here, an electrochemical compartment is described in which the GDE includes a coating-free edge area and is connected to a supporting structure welded to an electrically conductive plate. Apart from the difficult replacement, this technology also involves the essential disadvantage that there is a great loss of active electrode surface area due to the existing welds, which causes a decrease in the efficiency of the electrolysis cell.
An alternative method of fixing the gas-diffusion electrodes is described in DE 101 52 792. Here, a method is described in which a gas-diffusion electrode is connected to the base structure of the electrolysis apparatus unit by means of a circumferential fold-type frame. As a mere clamping method, this method is more advantageous with regard to replaceability than that described in DE 103 30 232. However, as in this case as well, the frame and the base structure are connected by welding or soldering for the minimisation of ohmic losses, the disadvantage of difficult replacement and the loss of active electrode surface area due to welds still persist.
DE 103 21 681 A1 discloses seal assemblies for electrolysis cell arrangements, the seal assemblies comprising a first sheet surface and a second sheet surface as well as a first cord-like seal acting essentially as a spacer and a second cord-like seal, both provided at a certain distance between the first sheet surface and the second sheet surface. The arrangement inside the cell provides for one seal overlapping with the membrane, this being followed by a bore and the second seal being provided in the outer area of the electrolysis cell, with the second seal acting as a spacer only and being without sealing effect. The disadvantage involved is that it is not possible to ensure complete sealing in this way and leaks may occur due to the bores.
U.S. Pat. No. 4,721,555 A describes an electrochemical cell which is provided with a circumferential gasket frame in the overlapping area of membrane and cell frame, the gasket frame featuring a plurality of shaped sections. This solution also still involves the risk of leaks.
Therefore, it is the objective of the present invention to find a technical solution which first ensures adequate sealing especially of the gas room against the electrolyte room in order to prevent the electrolyte from reaching the electrolyte outlet via areas not adequately sealed such as the vertical edge areas between the gas-diffusion electrode and the insulating gasket frame, the electrolyte in such case not being available for the electrochemical reaction. In addition, the gas-diffusion electrode is also to be fixed in the electrochemical cell to ensure simple assembly and disassembly of the gas-diffusion electrode and to thus provide an as large active electrode surface area as possible for the electrochemical reaction. Furthermore, the sealing is to ensure the electrical insulation of the anode from the cathode in order to allow for the intended functioning mode of the electrochemical cell.